A magnetic inspection apparatus detects, by utilizing magnetism, defects such as internal and surface flaws and inclusion in a thin steel strip or a to-be-inspected object. It was reported that a magnetic inspection apparatus, in which a magnetic sensor array comprising linearly arranged magnetic sensors for detecting magnetic fluxes is built, is capable of successively detecting defects on a running thin steel strip over the entire width thereof (Published Unexamined Japanese Utility Model Application (PUJUMA No. 63-107849).
FIGS. 39 and 40 are schematic cross-sectional views showing, in different directions, the above-mentioned magnetic inspection apparatus for successively detecting the defects on the running thin steel strip. FIG. 41 is a side view showing the state in which the magnetic inspection apparatus is built in a support apparatus.
Referring to FIG. 41, a horizontal arm 12 is supported by a pair of spring members 13a and 13b within a frame 11 set on the floor of a room. Accordingly, the arm 12 is vertically movable. A stationary shaft 2 of the magnetic inspection apparatus is fixed at the center of the arm 12. A pair of guide rolls 14a and 14b for guiding a thin steel strip 10 on the outer peripheral surface of a hollow roll 1 are arranged on both sides of the frame 11.
In FIGS. 39 and 40, one end portion of the stationary shaft 2 penetrates the hollow roll 1 of a non-magnetic material along the center axis of the roll 1. The other end portion of the shaft 2 is fixed on the horizontal arm 12. The stationary shaft 2 is supported on the inner peripheral surfaces of both end portions of the hollow roll 1 by a pair of rolling bearings 3a and 3b such that the shaft 2 is situated along the center axis of the hollow roll 1. Accordingly, the hollow roll 1 is freely rotatable about the stationary shaft 2.
A magnetizing core 4c having a substantially U-cross section is fixed to the stationary shaft 2 by means of a support member 5 within the hollow roll 1, such that magnetic poles 4a and 4b of the core 4c are situated close to the inner peripheral surface of the hollow roll 1. A magnetizing coil 6 is wound around the magnetizing core 4c. Thus, the magnetizing core 4c and magnetizing coil 6 constitute a magnetizer 4. A magnetic sensor array 7 consisting of magnetic sensors 7a arranged linearly along the axis of the hollow roll 1 is fixed to the stationary shaft 2 between the magnetic poles 4a and 4b of the magnetizing core 4c.
A power cable 8 for supplying an excitation current to the magnetizing coil 6 and a signal cable 9 for taking out output signals from the magnetic sensors 7a of the magnetic sensor array 7 are led to the outside through the inside passage of the stationary shaft 2. Accordingly, the positions of the magnetizer 4 and magnetic sensor array 7 are fixed, and the hollow roll 1 rotates around the magnetizer 4 and magnetic sensor array 7 with a small gap.
when the outer peripheral surface of the hollow roll 1 of the magnetic inspection apparatus with the above structure is pressed on one side surface of the thin steel strip 10 under a predetermined pressure which runs, for example, in a direction a, the hollow roll 1 rotates in a direction b since the stationary shaft 2 is fixed on the horizontal arm 12.
In the above magnetic inspection apparatus, when an excitation current is supplied to the magnetizing coil 6, a closed magnetic path is formed by the magnetic poles 4a and 4b of the magnetizing core 4c and the running thin steel strip 10. If there is an internal or surface defect of the thin steel strip, the magnetic path in the thin steel strip is disturbed and a leakage magnetic flux occurs. The leakage magnetic flux is detected by the magnetic sensor 7a which constitutes a part of the magnetic sensor array 7 and faces the location of the defect. A signal corresponding to the defect is output from this magnetic sensor 7a.
The level of this output detection signal corresponds to the magnitude of the internal or surface defect of the thin steel strip 10. Thus, by measuring the level of the output signal, the width, directional position and magnitude of the internal or surface defect of the steel strip 10 can be determined.
However, regarding the above-described magnetic inspection apparatus, there are the following problems to be solved.
When a small defect of the object such as the thin steel strip 10 is detected, the S/N does not basically increase unless the magnetic force is adequate.
In order to solve such a problem, there is an idea that the excitation current to the magnetizing coil 6 of the magnetizer 4 is increased to intensify the magnetic field forming in the thin steel strip 10. The greater the magnetic flux in the steel strip 10, the higher the value of the leakage magnetic flux due to the defect.
In general, as shown in FIG. 42, when the magnetic poles 4a and 4b are situated near the thin steel strip 10, magnetic force lines extend from the pole 4a to the pole 4b through a magnetic gap, the thin steel plate 10 and another magnetic gap. Since the thin steel strip 10 is formed of a ferromagnetic material, magnetic fluxes do not leak out of the thin steel strip 10 while passing through the strip 10 if no defect is present in the strip 10.
However, as stated above, if the magnetic field applied to the thin steel strip 10 is increased so as to obtain a leakage magnetic flux of a sufficiently high signal level when a defect is present, the strip 10 is magnetically saturated, as shown in FIG. 42. AS a result, a large floating magnetic flux 15 occurs even in a defect-free portion. The actual value of the floating flux 15 is extremely high, e.g. several Gauss to several-ten Gauss.
In addition, it is experimentally confirmed that the variation in a vertical component of the floating flux 15 depends greatly on the speed of the thin steel strip 10. FIG. 43 shows the relationship between the output voltage and the speed of the thin steel strip 10, in the case where the vertical floating magnetic flux which is the flux outside the thin steel strip 10 in the defect-free portion was measured with the sensitivity of the magnetic sensors 7a lowered intentionally. As shown in FIG. 43, the output voltage rises as the speed of the thin steel strip 10 increases. Accordingly, the variation in vertical component of the floating magnetic flux 15 rises in accordance with the increase in speed.
Since the floating magnetic flux 15 is always generated, the leakage magnetic flux due to a defect is superimposed on the floating magnetic flux, when the defect is present on the thin steel strip 10. In addition, the floating magnetic flux is greater than the leakage magnetic flux. Each magnetic sensor 7a detects the floating flux, as shown in FIG. 42, and the leakage flux, simultaneously.
The same phenomenon occurs in the case where the magnetic sensor 7a is situated on the magnetic pole side of the thin steel strip 10, as indicated by a solid line in FIG. 42, and in the case where the magnetic sensor 7a is situated on that side of the strip 10 opposite to the magnetic poles 4a and 4b, as indicated by a broken line.
On the other hand, in order to detect the defect of the thin steel strip 10 with high precision, it is necessary to increase the sensitivity of the magnetic sensors 7a. However, as stated above, the variation component of the leakage magnetic flux due to the defect is superimposed on the high-level floating flux in the defect-free portion. Thus, if the high-sensitivity magnetic sensor array 7 is used, the magnetic sensors are saturated by the floating flux because of their narrow dynamic range, and the leakage magnetic flux due to the defect cannot be detected with high precision.